Why Root

From Android Wiki

You never know

The good thing about having root is you can go back if you want. The same is not true of not having root. We do not know what steps Google/Tmobile will take to rid the end users of root in the future. For all we know the next OTA will remove the keys that allows the RC29 downgrade method to work and you could be stuck on a non rooted G1 forever. If you have root and for some reason it does not work for you there is always the option of going back.

You can make a folder on your sdcard and backup all of your .apk files to your sdcard (helps if an author decides to "upgrade" you to a version that requires you to pay to use the version you just had)